By now many of you have probably seen images of green-glowing zebrafish, or pigs whose snout & trotters glow in the dark. In both cases the animals are genetically modified and are expressing a fluorescent protein originally sourced from a jellyfish. (The body form of a jellyfish is a medusa, while that of sea anemones & their freshwater relative, Hydra, is called a polyp.) There are a range of these proteins, which collectively belong to a group called the Green Fluorescent Proteins (what else?), and while a wide range of jellyfish produce them1 there are only occasional reports of glowing polyps.
However, in a paper just published in the open-access journal PLoS ONE, Andrey Prudkovsky & his colleagues describe finding tiny (~1.5 mm) fluorescent polyps living in the Red Sea. More specifically, growing in colonies on the shells of small gastropods, a relationship described as epibiotic. The snails are active at night on the sandy seabed, and the researchers noted that the little gastropods buried themselves in the sand when a torch shone on them. They also noticed that the snails, as they moved about in the moonlight, were covered with tiny pinpoints of green light.
Where fluorescence has been described in other polyps, it's mostly been in the 'stalks' of the little animals, but in all the polyps Prudkovsky's team studied, the intense green glow came from a region known as the hypostome - the region around the animal's mouth & encircled by its tentacles. Because both the intensity and the site at which the proteins are expressed is so unusual, the researchers suggest that this could be a useful taxonomic characteristic, given that it's hard to tell one colonial polyp species from another.
They also speculate on the adaptive significance of a polyp having a green glow around its mouth, suggesting that
[f]luorescence in the hypostome of Cytaeis sp. has probable ecological significance as prey are likely to be attracted to the tentacles and mouth of the polyps
although I do feel that until there's actual observational evidence of this happening, it's a little like an evolutionary just-so story. But isn't the combination of little snails and glowing polyps rather beautiful?
Fig 3 From Prudkovsky et al., 2016: Hydroid polyps of Cytaeis sp. from the Saudi Arabian Red Sea, scale bar 2mm; (A) fluorescence of living polyps on the shell of the gastropod Nassarius margaritifer; (B) polyps on the shell of a N.margaritifer specimen, scale bar 2mm; (C) close-up of polyps, scale bar 0.5mm. doi:10.1371/journal.pone.0146861.g003
1 And also in comb jellies, marine arthropods, and
cephalopods cephalochordates (thanks to herr doktor bimler for picking up the evidence of my brainfade).
Prudkovsky AA, Ivanenko VN, Nikitin MA, Lukyanov KA, Belousova A, Reimer JD, et al. (2016) Green Fluorescence of Cytaeis Hydroids Living in Association with Nassarius Gastropods in the Red Sea. PLoS ONE 11(2): e0146861. doi:10.1371/journal.pone.0146861